Rare metals (for example, cobalt (Co), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), cadmium (Cd), lanthanum (La), cerium (Ce), neodymium (Nd), europium (Eu), terbium (Tb), mercury (Hg), uranium (U), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os)) are essential for our daily life. These metals are used in various products including modern precision equipment such as catalyst for automobile, fuel cell, and ultrahigh-strength magnet. Japan has been heavily dependent on imports for these metals; so, in view of stable supply of resources and environmental protection, recycle of these rare metals is an important technology.
For recycling of rare metals, conventionally, various solvent extraction methods from rare-metal-containing aqueous solution have been used and various extractants have been developed and used. For example, Patent documents 1 and 2 disclose that cyclic phenol sulfides having particular structures are useful as rare metal extractants and they are effective to selectively extract rare metal(s).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2007-239066    Patent Document 2: JP-A No. 2007-239088